The invention relates to a method and an apparatus for estimating angles in a synchronous machine, in particular in a permanent-magnet synchronous machine.
In order to operate permanent-magnet synchronous machines which are used, for example, in electrical drive technology for electrically operated vehicles such as electric vehicles or hybrid vehicles, it is necessary to know the rotor angle, that is to say the position of the rotor of the synchronous machine with respect to its rotational movement around the rotor shaft. This measurement variable is needed to match the generation of torque to the actual rotor position.
The rotor angle may be determined, for example, with the aid of angle sensors. However, such angle sensors, in particular angle sensors with a high resolution, may give rise to considerable costs when producing electrical drive systems. Therefore, cost-effective angle sensors with a correspondingly lower angular resolution are often used. The document DE 10 2007 052 365 A1, for example, discloses a method for determining a rotor angle of a synchronous machine with the aid of a low-resolution angle sensor and a Kalman filter.
Alternatively, instead of measuring the rotor angle using angle sensors, it is possible to carry out an estimation on the basis of current and/or voltage sensors. In this case, it is possible to use so-called active methods, that is to say methods in which an additional voltage signal is modulated onto the supply voltage applied to the machine and currents induced by this additional voltage signal in the machine are measured. With a suitable design of the machine, that is to say with sufficient magnetic anisotropy for example, the instantaneous rotor angle can be inferred from the measured currents. For example, the document Linke, M., et al.: “Sensorless position control of Permanent Magnet Synchronous Machines without Limitation at Zero Speed”, IEEE IECON '02 1, 2002, pages 674-679, discloses sensorless methods in which radio-frequency signals are injected into the supply voltage of the machine, and in which response signals induced by the radio-frequency signals in the machine are measured and allow the rotor angle to be estimated.
However, the known methods for determining the rotor angle are suitable only for particular restricted speed ranges. For example, safety requirements which are imposed on the electrical drive of an electric vehicle or hybrid vehicle cannot always be ensured when using sensorless methods. On the other hand, methods with low-resolution angle sensors are inferior to other methods in low speed ranges on account of the insufficient measurement data.
There is a need for solutions for determining rotor angles in a synchronous machine which make it possible to regulate torques in a more robust, reliable and powerful manner in all speed ranges of the synchronous machine without having to resort to cost-intensive high-resolution angle sensors.